Lovely smooth shading, Guido. There's no need to send this again...but... they could be even darker :o)

Assume for now that your drawing has one very dark tone or black in it. As soon as that dark tone has been drawn you immediately determine every other tone within the drawing. If that "dark" is light you will force yourself to work with a palette of light greys. Unless it was intentional, that usually leads to a flat drawing. Setting a wide range of tones, by making your darks dark will help with three-dimensional rendering and add visual impact to your work.

In the attached image below (just a quick demo from my last workshop) the left-hand copy is the same as the original. You can see in the right-hand copy, with the contrast reduced, that it loses life and a strong sense of roundness. That's what happens if your darks are too light.